I find in my garden that montana and alpina get excited and juicy and full of new foliage and flwoer buds just in time to be knocked out by a severe late frost and we always get a -15 or so in late March or even early April that does for them. I grow lots of clematis but not the cirrhosa, alpina, macro petala or montana forms and I now make sure when buying new ones that they can cope down to -25C as we get that quite a lot in recent winters.